It turned out out solid fuel rockets are operationally more practical for the use cases like air defence, long range missiles that are ballistic instead of cruising in the atmosphere and so on. And jet engines are more efficient for subsonic cruise missiles. Ramjets are still used in some missiles like the long range mach 3 air-to-air Meteor.

ICBMs can go faster than this already but as I understand it they go higher allowing for earlier detection and they follow a more predictable trajectory which makes interception more realistic.

I find super fast missiles far scarier than advanced AI. I suppose they maintain the "mutually assured destruction" which might be the main reason there hasn't been a nuclear war since WW2, but it's not a huge comfort.

Seems more likely that Japan is designing this engine for a hypersonic cruise missile program, and the passenger aircraft concept is somewhat of a cover.

IMO, there is no point in a Mach-5 Aircraft (other than cruise missiles). There is potentially some point in Mach 2-3 aircraft, (not that we have ever made them commercially viable) but at the boundary to hypersonic, you might as well just switch to a suborbital hop concept.

A suborbital hop gets you to anywhere in the world within ~90min, avoids issues of supersonic overflight and you don't need to worry about the massive engineering issues caused by sustaining hypersonic flight. And as a bonus, the passengers get a hour of weightlessness.

From an environmental point of view, I hope this won't materialise for some time.

I'd prefer either: just as uncomfortable or even more so but very fast, like multiple mach numbers fast. I live in the Midwest and go to the West Coast often, and it usually takes 5 hours -- I'd cram into a less comfortable seat for 2 hours instead to get there at mach 2.5.

OR... airships. Big cushy seats. A lounge with fast wifi and desks. A coffee shop. You can walk around. The trip takes all day, or overnight with a sleeping cabin. Trains could fill this niche too, but airships could go overseas.

Go faster or go roomier.

> A scramjet is a variant of a ramjet airbreathing jet engine in which combustion takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to compress the incoming air forcefully before combustion, but whereas a ramjet decelerates the air to subsonic velocities before combustion using shock cones, a scramjet has no shock cone and slows the airflow using shockwaves produced by its ignition source in place of a shock cone - Wikipedia

I always thought it was an underutilized design that could be improved for practical applications.

Improved enough, it could become cheaper and more environmentally-friendly than current aviation whereas regular supersonic jets are never going to achieve that.

It is not the conical nose or leading edges that are the show stopper problem(s). There the shockwave generally does not touch the craft. The internal shockwaves that touch the walls of the engine ducting are. The heat loading and heat soak ability on those shockwave impingement sites will limit the duration of hypersonic travel.

Hypersonic travel through the atmosphere is easy, a problem solved in the 1950s. Be conical and carry your oxygen internally. Hypersonic travel that is air-breathing is an entirely different class of problem and I don't think it is anywhere near to being solved.

The only silver lining is that at hypersonic speeds you don't need to be propulsive for very long to get anywhere.

> To handle that level of heat, engineers constructed an advanced thermal‑protection system that maintained the aircraft's interior near normal operating temperature, allowing the onboard avionics and control electronics to function normally.

Hindenburg 2.0 waiting to happen

No. By itself, a new hypersonic engine can't make 2-hour flights between Japan and the US a reality. We are not even close to being able to build an aircraft that can do that - we don't even have the materials for that. What seems "easier" (as in "less impossible") is a hypersonic glider design that enters a suborbital trajectory and does shuttle-like aerobraking while it glides to its destination, before reengaging propulsion prior to landing on an airstrip (because passenger planes need to be able to abort landings and do multiple attempts). Not sure how reverse thrust would work there - variable geometry rocket bells?